My Almarro 318B amp has recently developed a low level hum that comes and goes. I have swapped out all 4 tubes at one point or another and that does not change it. Once the hum stopped for a few days while I was breaking in a new paid (NOS) of 6C33C tubes. But it keeps coming and going. It gets a bit louder and then softer. If I tap the base of the amp, the hum stops. If I pick up the amp, it stops. If I knock the table where the amp sits, it stops.

I run the amp about 6-8 hours a day. Once a week I foget and leave it on all night.

It sounds like it could be some transformer hum since it goes away when you touch the amp on occasion. Not guaranteed to be the answer, but you might try tightening down the transformer bolts and or putting a dense heavy object on top of the transformers. You don't need any magic pebbles or crap like that, just a brick would do(doesn't have to be "magic" either), though not very pretty to look at. Place felt or rubber pads between the heavy object and transformer to keep from scratching it (beware that rubber may melt if those trannies get hot - use felt in that case). Use whatever you might have on hand and see if it improves and if it works you can look for something that floats your boat to put on there. Mapleshade makes brass "HeavyWeights" that they charge a bit for or you can go to a scrap yard and find some scrap brass or?

Jax2 - good thinking. If it is a transformer, is it humming because it is aging? Malfunctioning? Or is that just how transformers are? Can I address the source of the problem or just deal with the symptom? Thanks for your comment.

sometimes transformer hum can (as Jax2 said) just be the result of loose mounting bolts, sometimes not. Sometimes its just how that particular tranny is. Toroidal trannys are often less susceptible. One other cause sometimes is DC offset, IIRC.

Yes, as Swampwalker said, or rather, let me paraphrase; Hum Happens. Not a defect or necessarily worn out (Could be loose bolts, which you could address). Could also be a drop in voltage (I think when you get on 10% or so you could induce some hum). Try the solutions I offered and see if that works. I guess that does deal with the symptom, but if it's just a loose bolt you've dealt with the problem. Try the weights if the bolt doesn't work. Could be tubes (remote chance) too. I'd look at the trannies first - not as a defect, but just for something loose or vibrating. I'd strongly suspect that given that it stops when you touch or move them.

I know just enough about this stuff to sound like I know what I'm talking about, or rather, just enough to be dangerous. To try to address your question, and someone will have to correct me on this if I'm inaccurate, but I think a tranny could break down (some of the binding element they use inside) and that could have that effect of exacerbating hum, BUT, as I understand it, that would be over a greater period of time than an Almarro amp using 6C33C tubes could be old (in other words, not likely). Still could be vibrating. I have a friend who owned those amps for a while. I'll ask him and point him to this thread.

As Jax2 said I had this amp for a while but did not have any hum at all. So I don't have much to add. I contacted Almarro directly when i was looking for tubes and received replies the next day. You might try sending them an email to get more ideas for a solution (sales@almarro dot com).

The transformers are very secure on the chassis. With the suggestion of the current, I checked my circuit and found some other appliances (a laser printer and scanner) that were plugged into different plugs on the same circuit. I moved those and had no hum for two days. Now it is back but much much lower so I believe it is a current issue. I can live with the current hum but I'll send an email to Almarro to see if they have any ideas. Thanks to all for the great advice.

There are some power re-generators that have the potential to put out a constant clean output. I believe PS Audio offers several. You may be able to spend less money on installing a dedicated circuit with isolated ground, but if the fluctuations are coming from the grid (outside your house) that may not do much. No guarantees that either will 'solve' your problem. Did you try placing weights on the transformers when they are humming (I understand they are secure)? I'm not familiar with the amp, but there's a possibility it could be that the input or driver tube may be causing it...or perhaps a bad socket or something could shorting somewhere. Just a few more thoughts. Do both channels always hum together, or just a single channel? If the latter, is it always the same channel? Contacting Almarro is a great idea, or course. You'll find if you do a google search on "Almarro 318B Hum" you will find you get a handful of hits - At least one I noted said they'd had theirs replaced.

This may not apply to your situation. One of my Threshold SA10e mono amps developed a low level hum that would come and go. Recently the hum would appear more often and became louder. I removed the balanced cable and reconnected. The hum went away and came back within a few seconds. I felt the problem was with this connection. I tightened the two hex screws holding the XLR input in place to 10 inch lbs. and the hum was completely gone with no return visits. I did the same to the other amp and both amps are now dead quiet at zero volumn. I hope this helps someone.

I placed weights on the transfomers and indeed that solved the hum problem. Then I tried using springs under the amp to isolate it and that solved the hum problem without the weights. Thanks for all the great advice to solve the problem.